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CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="LIBPQ-CONNECT"
>31.1. Database Connection Control Functions</A
></H1
><P
>   The following functions deal with making a connection to a
   <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> backend server.  An
   application program can have several backend connections open at
   one time.  (One reason to do that is to access more than one
   database.)  Each connection is represented by a
   <TT
CLASS="STRUCTNAME"
>PGconn</TT
> object, which
   is obtained from the function <CODE
CLASS="FUNCTION"
>PQconnectdb</CODE
>,
   <CODE
CLASS="FUNCTION"
>PQconnectdbParams</CODE
>, or
   <CODE
CLASS="FUNCTION"
>PQsetdbLogin</CODE
>.  Note that these functions will always
   return a non-null object pointer, unless perhaps there is too
   little memory even to allocate the <TT
CLASS="STRUCTNAME"
>PGconn</TT
> object.
   The <CODE
CLASS="FUNCTION"
>PQstatus</CODE
> function should be called to check
   the return value for a successful connection before queries are sent
   via the connection object.

   <DIV
CLASS="WARNING"
><P
></P
><TABLE
CLASS="WARNING"
BORDER="1"
WIDTH="100%"
><TR
><TD
ALIGN="CENTER"
><B
>Warning</B
></TD
></TR
><TR
><TD
ALIGN="LEFT"
><P
>     On Unix, forking a process with open libpq connections can lead to
     unpredictable results because the parent and child processes share
     the same sockets and operating system resources.  For this reason,
     such usage is not recommended, though doing an <CODE
CLASS="FUNCTION"
>exec</CODE
> from
     the child process to load a new executable is safe.
    </P
></TD
></TR
></TABLE
></DIV
>

   </P><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>     On Windows, there is a way to improve performance if a single
     database connection is repeatedly started and shutdown.  Internally,
     libpq calls <CODE
CLASS="FUNCTION"
>WSAStartup()</CODE
> and <CODE
CLASS="FUNCTION"
>WSACleanup()</CODE
> for connection startup
     and shutdown, respectively.  <CODE
CLASS="FUNCTION"
>WSAStartup()</CODE
> increments an internal
     Windows library reference count which is decremented by <CODE
CLASS="FUNCTION"
>WSACleanup()</CODE
>.
     When the reference count is just one, calling <CODE
CLASS="FUNCTION"
>WSACleanup()</CODE
> frees
     all resources and all DLLs are unloaded.  This is an expensive
     operation.  To avoid this, an application can manually call
     <CODE
CLASS="FUNCTION"
>WSAStartup()</CODE
> so resources will not be freed when the last database
     connection is closed.
    </P
></BLOCKQUOTE
></DIV
><P>

   <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="LIBPQ-PQCONNECTDBPARAMS"
></A
><CODE
CLASS="FUNCTION"
>PQconnectdbParams</CODE
></DT
><DD
><P
>       Makes a new connection to the database server.

</P><PRE
CLASS="SYNOPSIS"
>PGconn *PQconnectdbParams(const char **keywords, const char **values, int expand_dbname);</PRE
><P>
      </P
><P
>       This function opens a new database connection using the parameters taken
       from two <TT
CLASS="SYMBOL"
>NULL</TT
>-terminated arrays. The first,
       <TT
CLASS="LITERAL"
>keywords</TT
>, is defined as an array of strings, each one
       being a key word. The second, <TT
CLASS="LITERAL"
>values</TT
>, gives the value
       for each key word. Unlike <CODE
CLASS="FUNCTION"
>PQsetdbLogin</CODE
> below, the parameter
       set can be extended without changing the function signature, so use of
       this function (or its nonblocking analogs <CODE
CLASS="FUNCTION"
>PQconnectStartParams</CODE
>
       and <CODE
CLASS="FUNCTION"
>PQconnectPoll</CODE
>) is preferred for new application
       programming.
      </P
><P
>       When <TT
CLASS="LITERAL"
>expand_dbname</TT
> is non-zero, the
       <TT
CLASS="PARAMETER"
>dbname</TT
> key word value is allowed to be recognized
       as a <TT
CLASS="PARAMETER"
>conninfo</TT
> string. See below for details.
      </P
><P
>       The passed arrays can be empty to use all default parameters, or can
       contain one or more parameter settings. They should be matched in length.
       Processing will stop with the last non-<TT
CLASS="SYMBOL"
>NULL</TT
> element
       of the <TT
CLASS="LITERAL"
>keywords</TT
> array.
      </P
><P
>       The currently recognized parameter key words are:

       <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="LIBPQ-CONNECT-HOST"
></A
><TT
CLASS="LITERAL"
>host</TT
></DT
><DD
><P
>           Name of host to connect to.
           If this begins with a slash, it specifies Unix-domain
           communication rather than TCP/IP communication; the value is the
           name of the directory in which the socket file is stored.  The
           default behavior when <TT
CLASS="LITERAL"
>host</TT
> is not specified
           is to connect to a Unix-domain
           socket in
           <TT
CLASS="FILENAME"
>/tmp</TT
> (or whatever socket directory was specified
           when <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> was built). On machines without
           Unix-domain sockets, the default is to connect to <TT
CLASS="LITERAL"
>localhost</TT
>.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-HOSTADDR"
></A
><TT
CLASS="LITERAL"
>hostaddr</TT
></DT
><DD
><P
>           Numeric IP address of host to connect to.  This should be in the
           standard IPv4 address format, e.g., <TT
CLASS="LITERAL"
>172.28.40.9</TT
>.  If
           your machine supports IPv6, you can also use those addresses.
           TCP/IP communication is
           always used when a nonempty string is specified for this parameter.
          </P
><P
>           Using <TT
CLASS="LITERAL"
>hostaddr</TT
> instead of <TT
CLASS="LITERAL"
>host</TT
> allows the
           application to avoid a host name look-up, which might be important
           in applications with time constraints. However, a host name is
           required for Kerberos, GSSAPI, or SSPI authentication
           methods, as well as for <TT
CLASS="LITERAL"
>verify-full</TT
> SSL
           certificate verification.  The following rules are used:
           <P
></P
></P><UL
><LI
><P
>              If <TT
CLASS="LITERAL"
>host</TT
> is specified without <TT
CLASS="LITERAL"
>hostaddr</TT
>,
              a host name lookup occurs.
             </P
></LI
><LI
><P
>              If <TT
CLASS="LITERAL"
>hostaddr</TT
> is specified without <TT
CLASS="LITERAL"
>host</TT
>,
              the value for <TT
CLASS="LITERAL"
>hostaddr</TT
> gives the server network address.
              The connection attempt will fail if the authentication
              method requires a host name.
             </P
></LI
><LI
><P
>              If both <TT
CLASS="LITERAL"
>host</TT
> and <TT
CLASS="LITERAL"
>hostaddr</TT
> are specified,
              the value for <TT
CLASS="LITERAL"
>hostaddr</TT
> gives the server network address.
              The value for <TT
CLASS="LITERAL"
>host</TT
> is ignored unless the
              authentication method requires it, in which case it will be
              used as the host name.
             </P
></LI
></UL
><P>
           Note that authentication is likely to fail if <TT
CLASS="LITERAL"
>host</TT
>
           is not the name of the server at network address <TT
CLASS="LITERAL"
>hostaddr</TT
>.
           Also, note that <TT
CLASS="LITERAL"
>host</TT
> rather than <TT
CLASS="LITERAL"
>hostaddr</TT
>
           is used to identify the connection in <TT
CLASS="FILENAME"
>~/.pgpass</TT
> (see
           <A
HREF="libpq-pgpass.html"
>Section 31.14</A
>).
          </P
><P
>           Without either a host name or host address,
           <SPAN
CLASS="APPLICATION"
>libpq</SPAN
> will connect using a
           local Unix-domain socket; or on machines without Unix-domain
           sockets, it will attempt to connect to <TT
CLASS="LITERAL"
>localhost</TT
>.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-PORT"
></A
><TT
CLASS="LITERAL"
>port</TT
></DT
><DD
><P
>           Port number to connect to at the server host, or socket file
           name extension for Unix-domain
           connections.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-DBNAME"
></A
><TT
CLASS="LITERAL"
>dbname</TT
></DT
><DD
><P
>          The database name.  Defaults to be the same as the user name.
         </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-USER"
></A
><TT
CLASS="LITERAL"
>user</TT
></DT
><DD
><P
>          <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> user name to connect as.
          Defaults to be the same as the operating system name of the user
          running the application.
         </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-PASSWORD"
></A
><TT
CLASS="LITERAL"
>password</TT
></DT
><DD
><P
>          Password to be used if the server demands password authentication.
         </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-CONNECT-TIMEOUT"
></A
><TT
CLASS="LITERAL"
>connect_timeout</TT
></DT
><DD
><P
>          Maximum wait for connection, in seconds (write as a decimal integer
          string). Zero or not specified means wait indefinitely.  It is not
          recommended to use a timeout of less than 2 seconds.
         </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-CLIENT-ENCODING"
></A
><TT
CLASS="LITERAL"
>client_encoding</TT
></DT
><DD
><P
>          This sets the <TT
CLASS="VARNAME"
>client_encoding</TT
>
          configuration parameter for this connection.  In addition to
          the values accepted by the corresponding server option, you
          can use <TT
CLASS="LITERAL"
>auto</TT
> to determine the right
          encoding from the current locale in the client
          (<TT
CLASS="ENVAR"
>LC_CTYPE</TT
> environment variable on Unix
          systems).
         </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-OPTIONS"
></A
><TT
CLASS="LITERAL"
>options</TT
></DT
><DD
><P
>           Adds command-line options to send to the server at run-time.
           For example, setting this to <TT
CLASS="LITERAL"
>-c geqo=off</TT
> sets the
           session's value of the <TT
CLASS="VARNAME"
>geqo</TT
> parameter to
           <TT
CLASS="LITERAL"
>off</TT
>.  For a detailed discussion of the available
           options, consult <A
HREF="runtime-config.html"
>Chapter 18</A
>.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-APPLICATION-NAME"
></A
><TT
CLASS="LITERAL"
>application_name</TT
></DT
><DD
><P
>           Specifies a value for the <A
HREF="runtime-config-logging.html#GUC-APPLICATION-NAME"
>application_name</A
>
           configuration parameter.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-FALLBACK-APPLICATION-NAME"
></A
><TT
CLASS="LITERAL"
>fallback_application_name</TT
></DT
><DD
><P
>           Specifies a fallback value for the <A
HREF="runtime-config-logging.html#GUC-APPLICATION-NAME"
>application_name</A
> configuration parameter.
           This value will be used if no value has been given for
           <TT
CLASS="LITERAL"
>application_name</TT
> via a connection parameter or the
           <TT
CLASS="ENVAR"
>PGAPPNAME</TT
> environment variable.  Specifying
           a fallback name is useful in generic utility programs that
           wish to set a default application name but allow it to be
           overridden by the user.
          </P
></DD
><DT
><A
NAME="LIBPQ-KEEPALIVES"
></A
><TT
CLASS="LITERAL"
>keepalives</TT
></DT
><DD
><P
>           Controls whether client-side TCP keepalives are used. The default
           value is 1, meaning on, but you can change this to 0, meaning off,
           if keepalives are not wanted.  This parameter is ignored for
           connections made via a Unix-domain socket.
          </P
></DD
><DT
><A
NAME="LIBPQ-KEEPALIVES-IDLE"
></A
><TT
CLASS="LITERAL"
>keepalives_idle</TT
></DT
><DD
><P
>           Controls the number of seconds of inactivity after which TCP should
           send a keepalive message to the server.  A value of zero uses the
           system default. This parameter is ignored for connections made via a
           Unix-domain socket, or if keepalives are disabled. It is only supported
           on systems where the <TT
CLASS="SYMBOL"
>TCP_KEEPIDLE</TT
> or <TT
CLASS="SYMBOL"
>TCP_KEEPALIVE</TT
>
           socket option is available, and on Windows; on other systems, it has no
           effect.
          </P
></DD
><DT
><A
NAME="LIBPQ-KEEPALIVES-INTERVAL"
></A
><TT
CLASS="LITERAL"
>keepalives_interval</TT
></DT
><DD
><P
>           Controls the number of seconds after which a TCP keepalive message
           that is not acknowledged by the server should be retransmitted.  A
           value of zero uses the system default. This parameter is ignored for
           connections made via a Unix-domain socket, or if keepalives are disabled.
           It is only supported on systems where the <TT
CLASS="SYMBOL"
>TCP_KEEPINTVL</TT
>
           socket option is available, and on Windows; on other systems, it has no
           effect.
          </P
></DD
><DT
><A
NAME="LIBPQ-KEEPALIVES-COUNT"
></A
><TT
CLASS="LITERAL"
>keepalives_count</TT
></DT
><DD
><P
>           Controls the number of TCP keepalives that can be lost before the
           client's connection to the server is considered dead.  A value of
           zero uses the system default. This parameter is ignored for
           connections made via a Unix-domain socket, or if keepalives are disabled.
           It is only supported on systems where the <TT
CLASS="SYMBOL"
>TCP_KEEPCNT</TT
>
           socket option is available; on other systems, it has no effect.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-TTY"
></A
><TT
CLASS="LITERAL"
>tty</TT
></DT
><DD
><P
>          Ignored (formerly, this specified where to send server debug output).
         </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-SSLMODE"
></A
><TT
CLASS="LITERAL"
>sslmode</TT
></DT
><DD
><P
>           This option determines whether or with what priority a secure
           <ACRONYM
CLASS="ACRONYM"
>SSL</ACRONYM
> TCP/IP connection will be negotiated with the
           server. There are six modes:

           <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="LITERAL"
>disable</TT
></DT
><DD
><P
>               only try a non-<ACRONYM
CLASS="ACRONYM"
>SSL</ACRONYM
> connection
              </P
></DD
><DT
><TT
CLASS="LITERAL"
>allow</TT
></DT
><DD
><P
>               first try a non-<ACRONYM
CLASS="ACRONYM"
>SSL</ACRONYM
> connection; if that
               fails, try an <ACRONYM
CLASS="ACRONYM"
>SSL</ACRONYM
> connection
              </P
></DD
><DT
><TT
CLASS="LITERAL"
>prefer</TT
> (default)</DT
><DD
><P
>               first try an <ACRONYM
CLASS="ACRONYM"
>SSL</ACRONYM
> connection; if that fails,
               try a non-<ACRONYM
CLASS="ACRONYM"
>SSL</ACRONYM
> connection
              </P
></DD
><DT
><TT
CLASS="LITERAL"
>require</TT
></DT
><DD
><P
>               only try an <ACRONYM
CLASS="ACRONYM"
>SSL</ACRONYM
> connection. If a root CA
               file is present, verify the certificate in the same way as
               if <TT
CLASS="LITERAL"
>verify-ca</TT
> was specified
              </P
></DD
><DT
><TT
CLASS="LITERAL"
>verify-ca</TT
></DT
><DD
><P
>               only try an <ACRONYM
CLASS="ACRONYM"
>SSL</ACRONYM
> connection, and verify that
               the server certificate is issued by a trusted
               certificate authority (<ACRONYM
CLASS="ACRONYM"
>CA</ACRONYM
>)
              </P
></DD
><DT
><TT
CLASS="LITERAL"
>verify-full</TT
></DT
><DD
><P
>               only try an <ACRONYM
CLASS="ACRONYM"
>SSL</ACRONYM
> connection, verify that the
               server certificate is issued by a
               trusted <ACRONYM
CLASS="ACRONYM"
>CA</ACRONYM
> and that the server host name
               matches that in the certificate
              </P
></DD
></DL
></DIV
><P>

           See <A
HREF="libpq-ssl.html"
>Section 31.17</A
> for a detailed description of how
           these options work.
          </P
><P
>           <TT
CLASS="LITERAL"
>sslmode</TT
> is ignored for Unix domain socket
           communication.
           If <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> is compiled without SSL support,
           using options <TT
CLASS="LITERAL"
>require</TT
>, <TT
CLASS="LITERAL"
>verify-ca</TT
>, or
           <TT
CLASS="LITERAL"
>verify-full</TT
> will cause an error, while
           options <TT
CLASS="LITERAL"
>allow</TT
> and <TT
CLASS="LITERAL"
>prefer</TT
> will be
           accepted but <SPAN
CLASS="APPLICATION"
>libpq</SPAN
> will not actually attempt
           an <ACRONYM
CLASS="ACRONYM"
>SSL</ACRONYM
>
           connection.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-REQUIRESSL"
></A
><TT
CLASS="LITERAL"
>requiressl</TT
></DT
><DD
><P
>           This option is deprecated in favor of the <TT
CLASS="LITERAL"
>sslmode</TT
>
           setting.
          </P
><P
>           If set to 1, an <ACRONYM
CLASS="ACRONYM"
>SSL</ACRONYM
> connection to the server
           is required (this is equivalent to <TT
CLASS="LITERAL"
>sslmode</TT
>
           <TT
CLASS="LITERAL"
>require</TT
>).  <SPAN
CLASS="APPLICATION"
>libpq</SPAN
> will then refuse
           to connect if the server does not accept an
           <ACRONYM
CLASS="ACRONYM"
>SSL</ACRONYM
> connection.  If set to 0 (default),
           <SPAN
CLASS="APPLICATION"
>libpq</SPAN
> will negotiate the connection type with
           the server (equivalent to <TT
CLASS="LITERAL"
>sslmode</TT
>
           <TT
CLASS="LITERAL"
>prefer</TT
>).  This option is only available if
           <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> is compiled with SSL support.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-SSLCERT"
></A
><TT
CLASS="LITERAL"
>sslcert</TT
></DT
><DD
><P
>           This parameter specifies the file name of the client SSL
           certificate, replacing the default
           <TT
CLASS="FILENAME"
>~/.postgresql/postgresql.crt</TT
>.
           This parameter is ignored if an SSL connection is not made.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-SSLKEY"
></A
><TT
CLASS="LITERAL"
>sslkey</TT
></DT
><DD
><P
>           This parameter specifies the location for the secret key used for
           the client certificate. It can either specify a file name that will
           be used instead of the default
           <TT
CLASS="FILENAME"
>~/.postgresql/postgresql.key</TT
>, or it can specify a key
           obtained from an external <SPAN
CLASS="QUOTE"
>"engine"</SPAN
> (engines are
           <SPAN
CLASS="PRODUCTNAME"
>OpenSSL</SPAN
> loadable modules).  An external engine
           specification should consist of a colon-separated engine name and
           an engine-specific key identifier.  This parameter is ignored if an
           SSL connection is not made.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-SSLROOTCERT"
></A
><TT
CLASS="LITERAL"
>sslrootcert</TT
></DT
><DD
><P
>           This parameter specifies the name of a file containing SSL
           certificate authority (<ACRONYM
CLASS="ACRONYM"
>CA</ACRONYM
>) certificate(s).
           If the file exists, the server's certificate will be verified
           to be signed by one of these authorities.  The default is
           <TT
CLASS="FILENAME"
>~/.postgresql/root.crt</TT
>.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-SSLCRL"
></A
><TT
CLASS="LITERAL"
>sslcrl</TT
></DT
><DD
><P
>           This parameter specifies the file name of the SSL certificate
           revocation list (CRL).  Certificates listed in this file, if it
           exists, will be rejected while attempting to authenticate the
           server's certificate.  The default is
           <TT
CLASS="FILENAME"
>~/.postgresql/root.crl</TT
>.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-REQUIREPEER"
></A
><TT
CLASS="LITERAL"
>requirepeer</TT
></DT
><DD
><P
>           This parameter specifies the operating-system user name of the
           server, for example <TT
CLASS="LITERAL"
>requirepeer=postgres</TT
>.
           When making a Unix-domain socket connection, if this
           parameter is set, the client checks at the beginning of the
           connection that the server process is running under the specified
           user name; if it is not, the connection is aborted with an error.
           This parameter can be used to provide server authentication similar
           to that available with SSL certificates on TCP/IP connections.
           (Note that if the Unix-domain socket is in
           <TT
CLASS="FILENAME"
>/tmp</TT
> or another publicly writable location,
           any user could start a server listening there.  Use this parameter
           to ensure that you are connected to a server run by a trusted user.)
           This option is only supported on platforms for which the
           <TT
CLASS="LITERAL"
>peer</TT
> authentication method is implemented; see
           <A
HREF="auth-methods.html#AUTH-PEER"
>Section 19.3.7</A
>.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-KRBSRVNAME"
></A
><TT
CLASS="LITERAL"
>krbsrvname</TT
></DT
><DD
><P
>           Kerberos service name to use when authenticating with Kerberos 5
           or GSSAPI.
           This must match the service name specified in the server
           configuration for Kerberos authentication to succeed. (See also
           <A
HREF="auth-methods.html#KERBEROS-AUTH"
>Section 19.3.5</A
> and <A
HREF="auth-methods.html#GSSAPI-AUTH"
>Section 19.3.3</A
>.)
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-GSSLIB"
></A
><TT
CLASS="LITERAL"
>gsslib</TT
></DT
><DD
><P
>           GSS library to use for GSSAPI authentication. Only used on Windows.
           Set to <TT
CLASS="LITERAL"
>gssapi</TT
> to force libpq to use the GSSAPI
           library for authentication instead of the default SSPI.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECT-SERVICE"
></A
><TT
CLASS="LITERAL"
>service</TT
></DT
><DD
><P
>           Service name to use for additional parameters.  It specifies a service
           name in <TT
CLASS="FILENAME"
>pg_service.conf</TT
> that holds additional connection parameters.
           This allows applications to specify only a service name so connection parameters
           can be centrally maintained. See <A
HREF="libpq-pgservice.html"
>Section 31.15</A
>.
          </P
></DD
></DL
></DIV
><P>

       If  any  parameter is unspecified, then the corresponding
       environment variable (see <A
HREF="libpq-envars.html"
>Section 31.13</A
>)
       is checked. If the  environment  variable is not set either,
       then the indicated built-in defaults are used.
      </P
><P
>        If <TT
CLASS="LITERAL"
>expand_dbname</TT
> is non-zero and
        <TT
CLASS="PARAMETER"
>dbname</TT
> contains an <TT
CLASS="SYMBOL"
>=</TT
> sign, it
        is taken as a <TT
CLASS="PARAMETER"
>conninfo</TT
> string in exactly the same way as
        if it had been passed to <CODE
CLASS="FUNCTION"
>PQconnectdb</CODE
>(see below). Previously
        processed key words will be overridden by key words in the
        <TT
CLASS="PARAMETER"
>conninfo</TT
> string.
      </P
><P
>        In general key words are processed from the beginning of these arrays in index
        order. The effect of this is that when key words are repeated, the last processed
        value is retained. Therefore, through careful placement of the
        <TT
CLASS="PARAMETER"
>dbname</TT
> key word, it is possible to determine what may
        be overridden by a <TT
CLASS="PARAMETER"
>conninfo</TT
> string, and what may not.
      </P
></DD
><DT
><A
NAME="LIBPQ-PQCONNECTDB"
></A
><CODE
CLASS="FUNCTION"
>PQconnectdb</CODE
></DT
><DD
><P
>       Makes a new connection to the database server.

</P><PRE
CLASS="SYNOPSIS"
>PGconn *PQconnectdb(const char *conninfo);</PRE
><P>
      </P
><P
>       This function opens a new database connection using the parameters taken
       from the string <TT
CLASS="LITERAL"
>conninfo</TT
>.
      </P
><P
>       The passed string can be empty to use all default parameters, or it can
       contain one or more parameter settings separated by whitespace.
       Each parameter setting is in the form <TT
CLASS="LITERAL"
>keyword = value</TT
>.
       Spaces around the equal sign are optional. To write an empty value,
       or a value containing spaces, surround it with single quotes, e.g.,
       <TT
CLASS="LITERAL"
>keyword = 'a value'</TT
>. Single quotes and backslashes
       within the value must be escaped with a backslash, i.e.,
       <TT
CLASS="LITERAL"
>\'</TT
> and <TT
CLASS="LITERAL"
>\\</TT
>.
      </P
><P
>       The currently recognized parameter key words are the same as above.
      </P
></DD
><DT
><A
NAME="LIBPQ-PQSETDBLOGIN"
></A
><CODE
CLASS="FUNCTION"
>PQsetdbLogin</CODE
></DT
><DD
><P
>       Makes a new connection to the database server.
</P><PRE
CLASS="SYNOPSIS"
>PGconn *PQsetdbLogin(const char *pghost,
                     const char *pgport,
                     const char *pgoptions,
                     const char *pgtty,
                     const char *dbName,
                     const char *login,
                     const char *pwd);</PRE
><P>
       </P
><P
>        This is the predecessor of <CODE
CLASS="FUNCTION"
>PQconnectdb</CODE
> with a fixed
        set of parameters.  It has the same functionality except that the
        missing parameters will always take on default values.  Write <TT
CLASS="SYMBOL"
>NULL</TT
> or an
        empty string for any one of the fixed parameters that is to be defaulted.
      </P
><P
>        If the <TT
CLASS="PARAMETER"
>dbName</TT
> contains an <TT
CLASS="SYMBOL"
>=</TT
> sign, it
        is taken as a <TT
CLASS="PARAMETER"
>conninfo</TT
> string in exactly the same way as
        if it had been passed to <CODE
CLASS="FUNCTION"
>PQconnectdb</CODE
>, and the remaining
        parameters are then applied as above.
      </P
></DD
><DT
><A
NAME="LIBPQ-PQSETDB"
></A
><CODE
CLASS="FUNCTION"
>PQsetdb</CODE
></DT
><DD
><P
>   Makes a new connection to the database server.
</P><PRE
CLASS="SYNOPSIS"
>PGconn *PQsetdb(char *pghost,
                char *pgport,
                char *pgoptions,
                char *pgtty,
                char *dbName);</PRE
><P>
     </P
><P
>      This is a macro that calls <CODE
CLASS="FUNCTION"
>PQsetdbLogin</CODE
> with null pointers
      for the <TT
CLASS="PARAMETER"
>login</TT
> and <TT
CLASS="PARAMETER"
>pwd</TT
> parameters.  It is provided
      for backward compatibility with very old programs.
     </P
></DD
><DT
><A
NAME="LIBPQ-PQCONNECTSTARTPARAMS"
></A
><CODE
CLASS="FUNCTION"
>PQconnectStartParams</CODE
><BR><CODE
CLASS="FUNCTION"
>PQconnectStart</CODE
><BR><CODE
CLASS="FUNCTION"
>PQconnectPoll</CODE
></DT
><DD
><P
>       
       Make a connection to the database server in a nonblocking manner.

</P><PRE
CLASS="SYNOPSIS"
>PGconn *PQconnectStartParams(const char **keywords,
                             const char **values,
                             int expand_dbname);

PGconn *PQconnectStart(const char *conninfo);

PostgresPollingStatusType PQconnectPoll(PGconn *conn);</PRE
><P>
      </P
><P
>       These three functions are used to open a connection to a database server such
       that your application's thread of execution is not blocked on remote I/O
       whilst doing so. The point of this approach is that the waits for I/O to
       complete can occur in the application's main loop, rather than down inside
       <CODE
CLASS="FUNCTION"
>PQconnectdbParams</CODE
> or <CODE
CLASS="FUNCTION"
>PQconnectdb</CODE
>, and so the
       application can manage this operation in parallel with other activities.
      </P
><P
>       With <CODE
CLASS="FUNCTION"
>PQconnectStartParams</CODE
>, the database connection is made
       using the parameters taken from the <TT
CLASS="LITERAL"
>keywords</TT
> and
       <TT
CLASS="LITERAL"
>values</TT
> arrays, and controlled by <TT
CLASS="LITERAL"
>expand_dbname</TT
>,
       as described above for <CODE
CLASS="FUNCTION"
>PQconnectdbParams</CODE
>.
      </P
><P
>       With <CODE
CLASS="FUNCTION"
>PQconnectStart</CODE
>, the database connection is made
       using the parameters taken from the string <TT
CLASS="LITERAL"
>conninfo</TT
> as
       described above for <CODE
CLASS="FUNCTION"
>PQconnectdb</CODE
>.
      </P
><P
>       Neither <CODE
CLASS="FUNCTION"
>PQconnectStartParams</CODE
> nor <CODE
CLASS="FUNCTION"
>PQconnectStart</CODE
>
       nor <CODE
CLASS="FUNCTION"
>PQconnectPoll</CODE
> will block, so long as a number of
       restrictions are met:
       <P
></P
></P><UL
><LI
><P
>          The <TT
CLASS="LITERAL"
>hostaddr</TT
> and <TT
CLASS="LITERAL"
>host</TT
> parameters are used appropriately to ensure that
          name and reverse name queries are not made. See the documentation of
          these parameters under <CODE
CLASS="FUNCTION"
>PQconnectdbParams</CODE
> above for details.
         </P
></LI
><LI
><P
>          If you call <CODE
CLASS="FUNCTION"
>PQtrace</CODE
>, ensure that the stream object
          into which you trace will not block.
         </P
></LI
><LI
><P
>          You ensure that the socket is in the appropriate state
          before calling <CODE
CLASS="FUNCTION"
>PQconnectPoll</CODE
>, as described below.
         </P
></LI
></UL
><P>
      </P
><P
>       Note: use of <CODE
CLASS="FUNCTION"
>PQconnectStartParams</CODE
> is analogous to
       <CODE
CLASS="FUNCTION"
>PQconnectStart</CODE
> shown below.
      </P
><P
>       To begin a nonblocking connection request, call <TT
CLASS="LITERAL"
>conn = PQconnectStart("<TT
CLASS="REPLACEABLE"
><I
>connection_info_string</I
></TT
>")</TT
>.
       If <TT
CLASS="VARNAME"
>conn</TT
> is null, then <SPAN
CLASS="APPLICATION"
>libpq</SPAN
> has been unable to allocate a new <TT
CLASS="STRUCTNAME"
>PGconn</TT
>
       structure. Otherwise, a valid <TT
CLASS="STRUCTNAME"
>PGconn</TT
> pointer is returned (though not yet
       representing a valid connection to the database). On return from
       <CODE
CLASS="FUNCTION"
>PQconnectStart</CODE
>, call <TT
CLASS="LITERAL"
>status = PQstatus(conn)</TT
>. If <TT
CLASS="VARNAME"
>status</TT
> equals
       <TT
CLASS="SYMBOL"
>CONNECTION_BAD</TT
>, <CODE
CLASS="FUNCTION"
>PQconnectStart</CODE
> has failed.
      </P
><P
>       If <CODE
CLASS="FUNCTION"
>PQconnectStart</CODE
> succeeds, the next stage is to poll
       <SPAN
CLASS="APPLICATION"
>libpq</SPAN
> so that it can proceed with the connection sequence.
       Use <CODE
CLASS="FUNCTION"
>PQsocket(conn)</CODE
> to obtain the descriptor of the
       socket underlying the database connection.
       Loop thus: If <CODE
CLASS="FUNCTION"
>PQconnectPoll(conn)</CODE
> last returned
       <TT
CLASS="SYMBOL"
>PGRES_POLLING_READING</TT
>, wait until the socket is ready to
       read (as indicated by <CODE
CLASS="FUNCTION"
>select()</CODE
>, <CODE
CLASS="FUNCTION"
>poll()</CODE
>, or
       similar system function).
       Then call <CODE
CLASS="FUNCTION"
>PQconnectPoll(conn)</CODE
> again.
       Conversely, if <CODE
CLASS="FUNCTION"
>PQconnectPoll(conn)</CODE
> last returned
       <TT
CLASS="SYMBOL"
>PGRES_POLLING_WRITING</TT
>, wait until the socket is ready
       to write, then call <CODE
CLASS="FUNCTION"
>PQconnectPoll(conn)</CODE
> again.
       If you have yet to call
       <CODE
CLASS="FUNCTION"
>PQconnectPoll</CODE
>, i.e., just after the call to
       <CODE
CLASS="FUNCTION"
>PQconnectStart</CODE
>, behave as if it last returned
       <TT
CLASS="SYMBOL"
>PGRES_POLLING_WRITING</TT
>.  Continue this loop until
       <CODE
CLASS="FUNCTION"
>PQconnectPoll(conn)</CODE
> returns
       <TT
CLASS="SYMBOL"
>PGRES_POLLING_FAILED</TT
>, indicating the connection procedure
       has failed, or <TT
CLASS="SYMBOL"
>PGRES_POLLING_OK</TT
>, indicating the connection
       has been successfully made.
      </P
><P
>       At any time during connection, the status of the connection can be
       checked by calling <CODE
CLASS="FUNCTION"
>PQstatus</CODE
>. If this call returns <TT
CLASS="SYMBOL"
>CONNECTION_BAD</TT
>, then the
       connection procedure has failed; if the call returns <CODE
CLASS="FUNCTION"
>CONNECTION_OK</CODE
>, then the
       connection is ready.  Both of these states are equally detectable
       from the return value of <CODE
CLASS="FUNCTION"
>PQconnectPoll</CODE
>, described above. Other states might also occur
       during (and only during) an asynchronous connection procedure. These
       indicate the current stage of the connection procedure and might be useful
       to provide feedback to the user for example. These statuses are:

       <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="LIBPQ-CONNECTION-STARTED"
></A
><TT
CLASS="SYMBOL"
>CONNECTION_STARTED</TT
></DT
><DD
><P
>           Waiting for connection to be made.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECTION-MADE"
></A
><TT
CLASS="SYMBOL"
>CONNECTION_MADE</TT
></DT
><DD
><P
>           Connection OK; waiting to send.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECTION-AWAITING-RESPONSE"
></A
><TT
CLASS="SYMBOL"
>CONNECTION_AWAITING_RESPONSE</TT
></DT
><DD
><P
>           Waiting for a response from the server.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECTION-AUTH-OK"
></A
><TT
CLASS="SYMBOL"
>CONNECTION_AUTH_OK</TT
></DT
><DD
><P
>           Received authentication; waiting for backend start-up to finish.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECTION-SSL-STARTUP"
></A
><TT
CLASS="SYMBOL"
>CONNECTION_SSL_STARTUP</TT
></DT
><DD
><P
>           Negotiating SSL encryption.
          </P
></DD
><DT
><A
NAME="LIBPQ-CONNECTION-SETENV"
></A
><TT
CLASS="SYMBOL"
>CONNECTION_SETENV</TT
></DT
><DD
><P
>           Negotiating environment-driven parameter settings.
          </P
></DD
></DL
></DIV
><P>

       Note that, although these constants will remain (in order to maintain
       compatibility), an application should never rely upon these occurring in a
       particular order, or at all, or on the status always being one of these
       documented values. An application might do something like this:
</P><PRE
CLASS="PROGRAMLISTING"
>switch(PQstatus(conn))
{
        case CONNECTION_STARTED:
            feedback = "Connecting...";
            break;

        case CONNECTION_MADE:
            feedback = "Connected to server...";
            break;
.
.
.
        default:
            feedback = "Connecting...";
}</PRE
><P>
      </P
><P
>       The <TT
CLASS="LITERAL"
>connect_timeout</TT
> connection parameter is ignored
       when using <CODE
CLASS="FUNCTION"
>PQconnectPoll</CODE
>; it is the application's
       responsibility to decide whether an excessive amount of time has elapsed.
       Otherwise, <CODE
CLASS="FUNCTION"
>PQconnectStart</CODE
> followed by a
       <CODE
CLASS="FUNCTION"
>PQconnectPoll</CODE
> loop is equivalent to
       <CODE
CLASS="FUNCTION"
>PQconnectdb</CODE
>.
      </P
><P
>       Note that if <CODE
CLASS="FUNCTION"
>PQconnectStart</CODE
> returns a non-null pointer, you must call
       <CODE
CLASS="FUNCTION"
>PQfinish</CODE
> when you are finished with it, in order to dispose of
       the structure and any associated memory blocks. This must be done even if
       the connection attempt fails or is abandoned.
      </P
></DD
><DT
><A
NAME="LIBPQ-PQCONNDEFAULTS"
></A
><CODE
CLASS="FUNCTION"
>PQconndefaults</CODE
></DT
><DD
><P
>       Returns the default connection options.
</P><PRE
CLASS="SYNOPSIS"
>PQconninfoOption *PQconndefaults(void);

typedef struct
{
    char   *keyword;   /* The keyword of the option */
    char   *envvar;    /* Fallback environment variable name */
    char   *compiled;  /* Fallback compiled in default value */
    char   *val;       /* Option's current value, or NULL */
    char   *label;     /* Label for field in connect dialog */
    char   *dispchar;  /* Indicates how to display this field
                          in a connect dialog. Values are:
                          ""        Display entered value as is
                          "*"       Password field - hide value
                          "D"       Debug option - don't show by default */
    int     dispsize;  /* Field size in characters for dialog */
} PQconninfoOption;</PRE
><P>
      </P
><P
>       Returns a connection options array.  This can be used to determine
       all possible <CODE
CLASS="FUNCTION"
>PQconnectdb</CODE
> options and their
       current default values.  The return value points to an array of
       <TT
CLASS="STRUCTNAME"
>PQconninfoOption</TT
> structures, which ends
       with an entry having a null <TT
CLASS="STRUCTFIELD"
>keyword</TT
> pointer.  The
       null pointer is returned if memory could not be allocated. Note that
       the current default values (<TT
CLASS="STRUCTFIELD"
>val</TT
> fields)
       will depend on environment variables and other context.  Callers
       must treat the connection options data as read-only.
      </P
><P
>       After processing the options array, free it by passing it to
       <CODE
CLASS="FUNCTION"
>PQconninfoFree</CODE
>.  If this is not done, a small amount of memory
       is leaked for each call to <CODE
CLASS="FUNCTION"
>PQconndefaults</CODE
>.
      </P
></DD
><DT
><A
NAME="LIBPQ-PQCONNINFOPARSE"
></A
><CODE
CLASS="FUNCTION"
>PQconninfoParse</CODE
></DT
><DD
><P
>       Returns parsed connection options from the provided connection string.

</P><PRE
CLASS="SYNOPSIS"
>PQconninfoOption *PQconninfoParse(const char *conninfo, char **errmsg);</PRE
><P>
      </P
><P
>       Parses a connection string and returns the resulting options as an
       array; or returns <TT
CLASS="SYMBOL"
>NULL</TT
> if there is a problem with the connection
       string.  This function can be used to extract
       the <CODE
CLASS="FUNCTION"
>PQconnectdb</CODE
> options in the provided
       connection string.  The return value points to an array of
       <TT
CLASS="STRUCTNAME"
>PQconninfoOption</TT
> structures, which ends
       with an entry having a null <TT
CLASS="STRUCTFIELD"
>keyword</TT
> pointer.
      </P
><P
>       Note that only options explicitly specified in the string will have
       values set in the result array; no defaults are inserted.
      </P
><P
>       If <TT
CLASS="LITERAL"
>errmsg</TT
> is not <TT
CLASS="SYMBOL"
>NULL</TT
>, then <TT
CLASS="LITERAL"
>*errmsg</TT
> is set
       to <TT
CLASS="SYMBOL"
>NULL</TT
> on success, else to a <CODE
CLASS="FUNCTION"
>malloc</CODE
>'d error string explaining
       the problem.  (It is also possible for <TT
CLASS="LITERAL"
>*errmsg</TT
> to be
       set to <TT
CLASS="SYMBOL"
>NULL</TT
> and the function to return <TT
CLASS="SYMBOL"
>NULL</TT
>;
       this indicates an out-of-memory condition.)
      </P
><P
>       After processing the options array, free it by passing it to
       <CODE
CLASS="FUNCTION"
>PQconninfoFree</CODE
>.  If this is not done, some memory
       is leaked for each call to <CODE
CLASS="FUNCTION"
>PQconninfoParse</CODE
>.
       Conversely, if an error occurs and <TT
CLASS="LITERAL"
>errmsg</TT
> is not <TT
CLASS="SYMBOL"
>NULL</TT
>,
       be sure to free the error string using <CODE
CLASS="FUNCTION"
>PQfreemem</CODE
>.
      </P
></DD
><DT
><A
NAME="LIBPQ-PQFINISH"
></A
><CODE
CLASS="FUNCTION"
>PQfinish</CODE
></DT
><DD
><P
>       Closes  the  connection to the server.  Also frees
       memory used by the <TT
CLASS="STRUCTNAME"
>PGconn</TT
> object.
</P><PRE
CLASS="SYNOPSIS"
>void PQfinish(PGconn *conn);</PRE
><P>
      </P
><P
>       Note that even if the server connection attempt fails (as
       indicated by <CODE
CLASS="FUNCTION"
>PQstatus</CODE
>), the application should call <CODE
CLASS="FUNCTION"
>PQfinish</CODE
>
       to free the memory used by the <TT
CLASS="STRUCTNAME"
>PGconn</TT
> object.
       The <TT
CLASS="STRUCTNAME"
>PGconn</TT
> pointer must not be used again after
       <CODE
CLASS="FUNCTION"
>PQfinish</CODE
> has been called.
      </P
></DD
><DT
><A
NAME="LIBPQ-PQRESET"
></A
><CODE
CLASS="FUNCTION"
>PQreset</CODE
></DT
><DD
><P
>       Resets the communication channel to the server.
</P><PRE
CLASS="SYNOPSIS"
>void PQreset(PGconn *conn);</PRE
><P>
      </P
><P
>       This function will close the connection
       to the server and attempt to  reestablish  a  new
       connection to the same server, using all the same
       parameters previously used.  This might be useful for
       error recovery if a working connection is lost.
      </P
></DD
><DT
><A
NAME="LIBPQ-PQRESETSTART"
></A
><CODE
CLASS="FUNCTION"
>PQresetStart</CODE
><BR><CODE
CLASS="FUNCTION"
>PQresetPoll</CODE
></DT
><DD
><P
>       Reset the communication channel to the server, in a nonblocking manner.

</P><PRE
CLASS="SYNOPSIS"
>int PQresetStart(PGconn *conn);

PostgresPollingStatusType PQresetPoll(PGconn *conn);</PRE
><P>
      </P
><P
>       These functions will close the connection to the server and attempt to
       reestablish a new connection to the same server, using all the same
       parameters previously used. This can be useful for error recovery if a
       working connection is lost. They differ from <CODE
CLASS="FUNCTION"
>PQreset</CODE
> (above) in that they
       act in a nonblocking manner. These functions suffer from the same
       restrictions as <CODE
CLASS="FUNCTION"
>PQconnectStartParams</CODE
>, <CODE
CLASS="FUNCTION"
>PQconnectStart</CODE
>
       and <CODE
CLASS="FUNCTION"
>PQconnectPoll</CODE
>.
      </P
><P
>       To initiate a connection reset, call
       <CODE
CLASS="FUNCTION"
>PQresetStart</CODE
>. If it returns 0, the reset has
       failed. If it returns 1, poll the reset using
       <CODE
CLASS="FUNCTION"
>PQresetPoll</CODE
> in exactly the same way as you
       would create the connection using <CODE
CLASS="FUNCTION"
>PQconnectPoll</CODE
>.
      </P
></DD
><DT
><A
NAME="LIBPQ-PQPINGPARAMS"
></A
><CODE
CLASS="FUNCTION"
>PQpingParams</CODE
></DT
><DD
><P
>       <CODE
CLASS="FUNCTION"
>PQpingParams</CODE
> reports the status of the
       server.  It accepts connection parameters identical to those of
       <CODE
CLASS="FUNCTION"
>PQconnectdbParams</CODE
>, described above.  It is not, however,
       necessary to supply correct user name, password, or database name
       values to obtain the server status.

</P><PRE
CLASS="SYNOPSIS"
>PGPing PQpingParams(const char **keywords, const char **values, int expand_dbname);</PRE
><P>

       The function returns one of the following values:

       <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="LIBPQ-PQPINGPARAMS-PQPING-OK"
></A
><TT
CLASS="LITERAL"
>PQPING_OK</TT
></DT
><DD
><P
>           The server is running and appears to be accepting connections.
          </P
></DD
><DT
><A
NAME="LIBPQ-PQPINGPARAMS-PQPING-REJECT"
></A
><TT
CLASS="LITERAL"
>PQPING_REJECT</TT
></DT
><DD
><P
>           The server is running but is in a state that disallows connections
           (startup, shutdown, or crash recovery).
          </P
></DD
><DT
><A
NAME="LIBPQ-PQPINGPARAMS-PQPING-NO-RESPONSE"
></A
><TT
CLASS="LITERAL"
>PQPING_NO_RESPONSE</TT
></DT
><DD
><P
>           The server could not be contacted.  This might indicate that the
           server is not running, or that there is something wrong with the
           given connection parameters (for example, wrong port number), or
           that there is a network connectivity problem (for example, a
           firewall blocking the connection request).
          </P
></DD
><DT
><A
NAME="LIBPQ-PQPINGPARAMS-PQPING-NO-ATTEMPT"
></A
><TT
CLASS="LITERAL"
>PQPING_NO_ATTEMPT</TT
></DT
><DD
><P
>           No attempt was made to contact the server, because the supplied
           parameters were obviously incorrect or there was some client-side
           problem (for example, out of memory).
          </P
></DD
></DL
></DIV
><P>

      </P
></DD
><DT
><A
NAME="LIBPQ-PQPING"
></A
><CODE
CLASS="FUNCTION"
>PQping</CODE
></DT
><DD
><P
>       <CODE
CLASS="FUNCTION"
>PQping</CODE
> reports the status of the
       server.  It accepts connection parameters identical to those of
       <CODE
CLASS="FUNCTION"
>PQconnectdb</CODE
>, described above.  It is not, however,
       necessary to supply correct user name, password, or database name
       values to obtain the server status.

</P><PRE
CLASS="SYNOPSIS"
>PGPing PQping(const char *conninfo);</PRE
><P>
      </P
><P
>       The return values are the same as for <CODE
CLASS="FUNCTION"
>PQpingParams</CODE
>.
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